Lasers are frequently used in material processing applications because laser beams can be focused to small spot sizes, thereby achieving the intensity and power density desired to process industrial-strength materials, such as metals. Exemplary processing applications include cutting, welding, surface modification, drilling and cladding. In a cutting operation, for example, it is generally desirable to focus a delivered laser beam to a small spot size so that a high intensity beam can be applied to the workpiece. This produces narrow kerf widths, high material removal rates and increased processing speed. Therefore, it is generally accepted that the higher the intensity of the applied beam, the faster the processing speed.
However, the relationship between intensity and processing speed breaks down as certain material parameters change, such as when material thickness increases. In cutting operations, for example, thicker materials require a minimum kerf width to ensure effective removal of the melt. This is because as cut speed decreases with increased material thickness, the decreased cut speed causes an increase in residence time in the material. If a laser beam of small spot size is used, this can generate significant evaporation of the material and produces undesirable side effects such as disruption of the metal flow down the kerf, rough and/or gouged cut edges, and even complete loss of penetration. The relationship between spot size and material thickness is more sensitive for oxygen-assisted cutting of mild steel because the exothermic combustion reaction of the steel and oxygen is a critical factor contributing to the cutting process.